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JohnGray Regular Visitor
Joined: Nov 22, 2004 Posts: 76
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Posted: Sat May 21, 2011 5:55 pm Post subject: Latitude and Longitude - has anyone done the maths? |
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I wonder whether anyone has worked out the difference in metres in the 5th decimal place of the usual figures for latitude and longitude in the UK.
For example, in the Motorway Junctions POIs, the values of
Longitude vary from about -4.54824° (West) to 1.15972° East
and the values of
Latitude vary from about 50.68351° to 56.38705° (North).
How much distance on the ground, roughly, does a change of 1 in the fifth decimal place represent for each direction (north-south and east-west)?
Taking the mid-point of the motorway system,
what's the distance between -1.69426° W and -1.69427° W Longitude
and between 53.53528° N and 53.53529° N Latitude?
And a supplementary question on these PGPSW POI files themselves: there are two files, one for Motorway Junction Entries and one for Motorway Junction Exits. Both use the same naming convention. Would it be necessary to suffix each row with "entry" or "exit", as appropriate, if the files were to be combined and uploaded to a Garmin Nuvi, so that they could be distinguished?
Thanks! |
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Skippy Pocket GPS Verifier
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
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Posted: Sat May 21, 2011 6:20 pm Post subject: Re: Latitude and Longitude - has anyone done the maths? |
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JohnGray wrote: | How much distance on the ground, roughly, does a change of 1 in the fifth decimal place represent for each direction (north-south and east-west)? |
About 1 meter or 3.3 feet North-South. Near enough for most of our measurements.
The difference of 0.00001 East-West varies according to how far you are North or South. Very roughly 1 meter. _________________ Gone fishing! |
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M8TJT The Other Tired Old Man
Joined: Apr 04, 2006 Posts: 10118 Location: Bexhill, South Sussex, UK
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Posted: Sat May 21, 2011 8:10 pm Post subject: |
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And multiply it by the cosine of your lattitude to find the distance E/W. So 0.00001 =about 3.3 feet N/S (skippy's calculation) and about 2.1 feet E/W at lat 50 |
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Oldboy Pocket GPS Moderator
Joined: Dec 08, 2004 Posts: 10642 Location: Suffolk, UK
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Posted: Sat May 21, 2011 9:06 pm Post subject: Re: Latitude and Longitude - has anyone done the maths? |
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JohnGray wrote: | And a supplementary question on these PGPSW POI files themselves: there are two files, one for Motorway Junction Entries and one for Motorway Junction Exits. Both use the same naming convention. Would it be necessary to suffix each row with "entry" or "exit", as appropriate, if the files were to be combined and uploaded to a Garmin Nuvi, so that they could be distinguished? | They are separate because they have different functions, but for the same junction.
The Entry points are set so that you can use the Motorway as a Waypoint, and come to, or join, it from any direction.
As an example, take M1 36N. If you wanted to travel on the M1 North, and go through Junction 36, you could come from any direction in a correct manner: North on the M1 from Junction 35 (or earlier), the A61 travelling North East, The A61, travelling South West, the A6195, travelling West and the A6135, travelling North. Travelling from any place would allow you to travel North on the M1 through/from Junction 36.
The Exit points are set so that you can use the Motorway Exit as a Waypoint, and also come to it from any direction.
The Exit point with the same Title, M1 36N, would mean that you would reach that Waypoint, on your way to your destination, whether you were travelling either way on the Motorway, or using any of the roads mentioned above.
You sometimes see the instructions to leave the Motorway at Junction nn. You may not be using the Motorway, but you will still pass through the correct Waypoint.
This Post explains the original reasons the POI was created. _________________ Richard
TT 910 V7.903: Europe Map v1045
TT Via 135 App 12.075: Europe Map v1120 |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Sun May 22, 2011 2:53 pm Post subject: Re: Latitude and Longitude - has anyone done the maths? |
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Oldboy wrote: | JohnGray wrote: | And a supplementary question on these PGPSW POI files themselves: there are two files, one for Motorway Junction Entries and one for Motorway Junction Exits. Both use the same naming convention. Would it be necessary to suffix each row with "entry" or "exit", as appropriate, if the files were to be combined and uploaded to a Garmin Nuvi, so that they could be distinguished? | They are separate because they have different functions, but for the same junction..... |
I don't know Garmin's that well, but I guess POI Loader must surely maintain the original POI category names when it compiles the single POI file to put on the machine, doesn't it?
So I'd have thought there was no problem there.
One stupid niggling thought though... and I hope I'm talking rubbish....
Haven't I read that Garmin use numbers in the POI names to produce their speed camera warnings? So the M40 junction 3 exit might produce 40mph and 3mph camera warnings?????
As for the Lat/Long question... I'd have tried the low tech solution...
Rather than do the complicated maths, I would just go to a spot on the TomTom (or Google maps) defined by some coordinates off the top of my head and mark it. Then repeat the exercise with the coordinates one number different, and measure the distance between them! Simples _________________ "Settling in nicely" ;-) |
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Daggers Lifetime Member
Joined: Jun 20, 2005 Posts: 1096 Location: Solihull, UK
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Posted: Sun May 22, 2011 4:28 pm Post subject: Re: Latitude and Longitude - has anyone done the maths? |
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Andy_P wrote: | One stupid niggling thought though... and I hope I'm talking rubbish....
Haven't I read that Garmin use numbers in the POI names to produce their speed camera warnings? So the M40 junction 3 exit might produce 40mph and 3mph camera warnings????? |
I think this only affected POI Categories, not individual POIs that were afected by this - e.g. a Category for Q8 fuel provided 8mph warnings every time you passed one of their petrol stations. _________________ Garmin DriveSmart 50 LMT-D |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Sun May 22, 2011 4:51 pm Post subject: |
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Of couse... silly me.
Ignore that bit of my last post! _________________ "Settling in nicely" ;-) |
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M8TJT The Other Tired Old Man
Joined: Apr 04, 2006 Posts: 10118 Location: Bexhill, South Sussex, UK
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Posted: Sun May 22, 2011 6:24 pm Post subject: Re: Latitude and Longitude - has anyone done the maths? |
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Andy_P wrote: | I would just go to a spot on the TomTom (or Google maps) defined by some coordinates off the top of my head and mark it. Then repeat the exercise with the coordinates one number different, and measure the distance between them! Simples | And what result did you get? |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Sun May 22, 2011 7:51 pm Post subject: |
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I said "i would go...."
But all right, just for you.
{time passes}
Yeah ok.... It's a bit too close to see, isn't it? Even on maximum zoom!
How about changing the THIRD decimal by one and dividing the distance on the ground by 100? Or doesn't it work like that?
{time passes}
That seems to give the right sort of result...
So 1.11 metres for 0.00001 deg of latitude in southern UK? _________________ "Settling in nicely" ;-) |
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JohnGray Regular Visitor
Joined: Nov 22, 2004 Posts: 76
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Posted: Sun May 22, 2011 8:09 pm Post subject: |
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Thanks for everyone's answers!
I have done some deep research (using Wikipedia's article on latitude and longitude!) and calculate
* for all longitudes (W-E), the latitude length(N-S) is 1.106 metres for each change of 0.00001° in latitude position
* for the most southerly latitude 50.68351°, the longitude length(W-E) is 0.705 metres for each change of 0.00001° in longitude position
* for the most northerly latitude 56.38705°, the longitude length(W-E) is 0.616 metres for each change of 0.00001° in longitude position
('most southerly' and 'most northerly' relate to the latitudes of the junctions found in the Motorway Junctions POI file)
Precision will depend minimally on the heights above sea level, and so on.
I do hope that this is correct! |
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M8TJT The Other Tired Old Man
Joined: Apr 04, 2006 Posts: 10118 Location: Bexhill, South Sussex, UK
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Posted: Sun May 22, 2011 11:08 pm Post subject: |
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A good way to work things out is to know that 1 degree of lattitude is 60 nautical miles (NM) (originally by definition). A NM is 1.85200 Km or if you prefer old money it's approximately 6,076 feet. It all becomes easy now. 1 degree = 6076 * 60 = 364,560ft. So the answer to the question is 364,560ft * 0.00001 = 3.64560ft. As I said before, multiply the distance N/S by the cosine of the latitude to give the distance E/W So a more accurate calculation of 0.00001 deg E/W at 50N is 2.34334651ft. |
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Skippy Pocket GPS Verifier
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
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Posted: Mon May 23, 2011 8:36 am Post subject: |
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M8TJT wrote: | A good way to work things out is to know that 1 degree of lattitude is 60 nautical miles (NM) (originally by definition). A NM is 1.85200 Km or if you prefer old money it's approximately 6,076 feet. It all becomes easy now. 1 degree = 6076 * 60 = 364,560ft. So the answer to the question is 364,560ft * 0.00001 = 3.64560ft. As I said before, multiply the distance N/S by the cosine of the latitude to give the distance E/W So a more accurate calculation of 0.00001 deg E/W at 50N is 2.34334651ft. |
Simples. _________________ Gone fishing! |
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M8TJT The Other Tired Old Man
Joined: Apr 04, 2006 Posts: 10118 Location: Bexhill, South Sussex, UK
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Posted: Mon May 23, 2011 9:11 am Post subject: |
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Andy_P wrote: | How about changing the THIRD decimal by one and dividing the distance on the ground by 100? Or doesn't it work like that? | Yes it does work that way, but you would first need a road that ran due N/S, or get involved in harder sums again But as the third decimal place is only 364 feet 6(ish) inches, it's hardly worth a drive. Or can't you walk 121 yards (110m) |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Mon May 23, 2011 12:49 pm Post subject: |
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M8TJT wrote: | Yes it does work that way, but you would first need a road that ran due N/S, or get involved in harder sums again |
No I didn't... See my pic.
Google maps, two accurate points, straight line measurement tool. Done.
_________________ "Settling in nicely" ;-) |
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